Abstract : The aim of this thesis is to show that a theory of perception cannot easily renounce to the concept of illusion without losing a part of its explanatory power.The problem of the opportunity to take recourse to the concept of illusions arises in view of the strong criticism against the notion of perceptual illusions within the frame of certain direct theories of perception.Nevertheless, the concept of illusion is not necessarily entangled with indirect approaches to perception and a characterization of illusory can be provided which is not based on the other concepts such as the concept of cognitive inference that are proper to indirect approaches. The proposed characterization of the notion of illusion is based on some characteristics of the perceptual experience with illusory phenomena: the existence of a violation of coherence which alerts the subject to the presence of an error, the robustness of the illusory experience intrasubjective and intersubjective systematicity and resilience to knowledge, and the presence of a reaction of surprise.The description of a special group of illusory perceptual phenomena that are characterized by violations of coherence, robustness and reaction of surprise provides the studies on perception with an instrument for acquiring an insight on perceptual mechanisms. These mechanisms include the one involved in the reactions of the perceptual system to the violation of coherence and the nature of expectations in perception. The study of certain illusions is also suitable for investigating the role that movement can play in perception and provides a better insight in the way movement and motor possibilities can shape the perceptual outcome. The notion of illusion hence presents a heuristic value also in relationship to views of perception such as the ecological and sensorimotor approaches that criticize the classical characterization of the notion of illusion.Once the notion of illusion is characterized, the role of illusory phenomena in the cognitive functioning in general can be better understood. It is in relationship with the notions of coherence and coherence violation that illusory phenomena seem to play their functional role in the frame of cognitive functioning. In fact, the awareness of illusions alerts the subject to the presence of an error epistemic value by signaling the existence of a violation of coherence which seems to have a negative adaptive value.